In exchange, Rosneft will receive a 5 percent BP stake valued at $7.8 billion, according to Bloomberg. While the main purpose of the joint venture is to explore for oil off Russia's vast Arctic coast, the deal has many moving parts, Reuters reports.

In the Russian deal, it was Freshfields corporate partner Sebastian Lawson and associate Sergei Diyachenko who advised Rosneft on the strategic joint venture agreement with BP, Legal Week reports. (For an earlier transaction of similar size, a $15 billion loan-for-oil deal with the Chinese government, Rosneft turned to Baker Botts for outside counsel.)

The London Evening Standard reports that the joint venture between BP and Rosneft left the major international investment banks on the sidelines, and that fees from the deal could exceed $15 million apiece for Freshfields and Linklaters.